1. Field of the Invention
The present invention relates to a method of manufacturing a liquid crystal display device, and more particularly, to an exposure method for a liquid crystal display device enabling to prevent mask misalignment.
2. Discussion of the Related Art
In general, a liquid crystal display device includes lower and upper glass substrates with a liquid crystal layer therebetween. On the lower glass substrate, a plurality of gate lines is arranged in one direction to leave a predetermined interval between each other and a plurality of data lines with a predetermined interval between them is arranged in another direction perpendicular to the gate to define a plurality of matrix type pixel areas. A plurality of pixel electrodes is formed in the pixel areas, respectively. A plurality of thin film transistors is formed adjacent to intersections of the gate and data lines in the pixel areas, respectively. The thin film transistors switch signals of data lines to corresponding pixel electrodes in accordance with signals from the gate lines, respectively.
On the upper glass substrate, a black matrix layer is positioned to prevent light from transmitting through portions of the upper glass substrate except in the pixel areas defined by the black matrix layer. A color filter layer for realizing colors is positioned in the pixel areas adjacent the black matrix layer. A common electrode is formed across the entire surface of the upper glass substrate including the color filter layer and black matrix layer. A liquid crystal layer is positioned between the lower and upper glass substrates by an injection process.
Such a liquid crystal display device is manufactured by forming a plurality of unit panels on a large-sized glass substrate simultaneously. At this time, each unit panel is prepared by deposition and patterning processes. Subsequently, a bonding process is performed, and then the unit panels are cut apart.
An example of a process for fabricating a cell of a unit panel on a lower substrate is explained as follows. First, a gate line having a gate electrode is formed on the lower substrate using a gate electrode pattern mask. Typically, the lower substrate is glass. A gate insulating layer is deposited on the gate line, gate electrode and the surface of the substrate. A semiconductor layer is formed on the gate insulating layer over the gate electrode using a semiconductor layer pattern mask. A data line having a source electrode connecting to one end of the semiconductor layer is formed in a direction perpendicular to the gate line. A drain electrode is formed on the other end of the semiconductor layer and is separated from the source electrode. The source and drain electrode are formed using a source/drain electrode pattern mask. A passivation layer is then deposited on the data line, source electrode, drain electrode, semiconductor layer and the entire surface of the gate insulating layer. A contact hole is formed in the passivation layer to expose the drain electrode using a contact hole pattern mask. A pixel electrode is formed on the passivation layer. The pixel electrode connects to the drain electrode via the contact hole using a pixel electrode pattern mask.
In order to form the gate electrode, semiconductor layer, source/drain electrodes or pixel electrode for each of the cells in a unit panel, the processes of depositing a material and then patterning the material across the entire surface of the unit panel are required. Such depositing and patterning processes are either concurrently or subsequently accompanied with thermal processes. After the material is deposited on the substrate, the surface of the deposited material is cleaned. A photoresist is then coated on the surface of the deposited material. Exposure and development are carried out on the photoresist using a mask, such as the gate electrode pattern mask, to obtain the desired pattern of material. The deposited material is then etched using the patterned photoresist as a mask. Subsequently, the remaining photoresist is removed. Typically, the photoresist is a material that can be dissolved in a developing agent selectively since the molecular configuration of the photoresist is changed by a light during exposure. For example, the photoresist can be comprised of a solvent, polymer and light sensitizer.
To correctly etch the material, the mask for exposure should be aligned correctly with the substrate on which the material is deposited so that the material will have the pattern as designated by the mask at the desired location. However, the above-explained unit panels are formed simultaneously on a large-sized sheet of glass that is subsequently cut into unit panels or individual liquid crystal devices. Because of high temperatures involved in forming the thin film transistor array and the type of large-sized sheet of glass used as the lower substrate, the lower glass substrate can contracts after a high temperature process or thermal process in the process of fabricating the thin film array on the large-sized sheet of glass. The contraction can cause misalignment of masks in subsequent process steps.
As discussed above, there are at least five masks required to form the cells of a thin film transistor array with a pixel electrode on the lower glass substrate in a unit panel. Typically, a material is formed by a deposition for all of the unit panels on the large-sized sheet of glass at the same time. The material can be deposited at a high temperature or subjected to a subsequent thermal treatment process to properly form the material. Typically, a photoresist is deposited on all of the unit panels of the large-sized sheet of glass at the same time. To pattern the photoresist, a stepper is used such that the cells of a unit panel are exposed together using a single mask. The single mask is repeatedly used to sequentially expose other unit panels on the same large-sized sheet of glass. In the alternative, a single large-sized mask that has patterns for all of the unit panels can be used to pattern all of the unit panels at the same time.
The high temperature processes used in forming a deposited material allows the molecular structure of the large-sized sheet of glass to realign such that the large-sized sheet of glass contracts. Since the masks for the fabrication of each unit panel is formed for the size of a unit panel""s lower substrate before the high temperature processes, the contraction causes a misalignment between the mask and the lower substrate for each unit panel on the large-sized sheet of glass. Thus, in the case of a mask for a single unit panel that is used in a stepper, the misalignment becomes pronounced as the stepper moves across the large-sized sheet of glass.
The exposure process of the liquid crystal display fabrication, in which a plurality of the panels are simultaneously manufactured on the single large-sized sheet of glass, according to the related art will be explained with reference to FIGS. 1, 2 and 3 as follows. FIG. 1 is a layout of a plurality of unit panels arranged on a large-sized sheet of glass. Referring to FIG. 1, a plurality of unit panels 1-1, 1-2, . . . , 5-4, 5-5 is arranged on a central portion of a large-sized sheet of glass 10. Alignment keys 2, 3, 4, and 5 are formed on corners of the glass substrate 10, respectively. The alignment keys 2, 3, 4, and 5 are for alignment of a stepper system (not shown) that repeatedly uses the same mask to pattern each of the unit panels. The process step of the stepper system that individually exposes a unit panel with the mask is called a shot. The stepper system, for example, exposes unit panel 1-1 in a first shot, and then moves up to expose unit panel 1-2 in a second shot and so on until the first column is complete. The rest of the columns are then similarly exposed, one after another.
Initially, the stepper system aligns on the four alignment keys 2, 3, 4 and 5 on the corners of the large-sized sheet of glass. Unfortunately, after a thermal process has been carried out at a high temperature, the glass of the large-sized sheet of glass contracts such that the distance between the alignment keys contracts. Hence, a subsequent mask for a unit panel will not be properly aligned if the stepper system again aligns to the four alignment keys 2, 3, 4 and 5 on the corners of the large-sized sheet of glass. If the exposure process for the next patterning process is carried out by aligning the stepper system with the alignment keys, misalignment will occur by a percentage corresponding to the percentage that the sheet of glass contracted.
FIG. 2 is a diagram for illustrating misalignment of a mask in a related art exposure process for manufacturing a liquid crystal display device. Typically, a stepper system starts at one corner and ends at an opposite corner. Referring to FIG. 2, when a mask of a stepper system is aligned based upon the alignment keys 2, 3 and 4 at the one corner adjacent to the alignment key 2 and an exposure process is carried out on each panel, the panel 1-1 adjacent to the alignment key 2 is properly aligned. However, misalignment increases for the unit panels as they become farther from the alignment key 2.
FIGS. 3 and 4 will be referred to in explaining how the misalignment of masks in the related art exposure process for manufacturing a liquid crystal display device progressively increases. FIG. 3 is a plan view of a first patterned material created by a related art exposure process. FIG. 4 is a plan view of a second pattern in relation to the first pattern of FIG. 3 created by the related art exposure process after the substrate was subjected to a high temperature process.
Referring to FIG. 3, a first material is deposited over each of the exemplary regions 6 on a substrate 7 formed of glass, and patterned by exposure and development into a first pattern 8. Referring to FIG. 4, a second material is deposited on the substrate 7 with a thermal process, and is patterned into a second pattern 9. When the thermal process was performed during the formation of the second pattern 9, the substrate 7 contracted. In this example, the mask alignment in the stepper is based upon the second alignment key 2 at the lower left corner. The second material is patterned in each of the exemplary regions 6 with the stepper by photolithography and etch processes that pattern the second material 9 into the shapes shown in FIG. 4. The position of the second pattern 9 near the alignment key 2 is correct. However, as the stepper proceeds in exposing exemplary regions in either the vertical or horizontal directions, the misalignment of the mask for each exemplary region 6 increases as the stepper makes exposures farther from the alignment key 2.
Accordingly, the present invention is directed to an exposure method for a liquid crystal display device that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide an exposure method for a liquid crystal display device to prevent misalignment of a mask.
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objectives and other advantages of the invention may be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these objects and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, an exposure method for a liquid crystal display devices formed as a plurality of unit panels on a large-sized sheet of glass using a stepper includes the steps of recording benchmark measurements of the large-sized sheet of glass, determining a change in dimension of the large-sized sheet of glass prior to a stepper positioning and exposing a mask on at least one unit panel of the plurality of unit panels, and compensating for the change in dimension by changing a position of an exposure for the at least one unit panel to a position other than where the stepper had previously exposed the at least one unit panel with a first mask.
In another aspect, an exposure method for a liquid crystal display devices formed as a plurality of unit panels on a large-sized sheet of glass using a stepper includes recording benchmark measurements of the large-sized sheet of glass, taking measurements of the large-sized sheet of glass after a high temperature process on the large-sized sheet of glass, determining contraction of the large-sized sheet of glass prior to a stepper positioning and exposing a mask on at least one unit panel of the plurality of unit panels, compensating for the contraction in dimension by changing the position of an exposure for the at least one unit panel to a position other than where the stepper had previously exposed the at least one unit panel with a first mask, performing an exposure process to the at least one unit panel with a second mask in the stepper at the changed exposure position, and performing an exposure process to the at least one unit panel with a third mask in the stepper at the changed exposure position.
It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory, and are intended to provide further explanation of the invention as claimed.